The "rosy" gene on chromosome 3 of Drosophila melanogaster is responsible for the biosynthesis of the enzyme xanthine dehydrogenase (XDH). A large body of genetic data has been accumulated for this differentiated organism, from the genetic standpoint. This proposal requests support for biochemical studies on xanthine dehydrogenase. These are designed to complement the genetic studies, and to extend our knowledge of the "rosy" region to the molecular level. Experiments proposed are designed to: 1. Determine the location of XDH structural variants within the polypeptide chain by peptide mapping. 2. Provide information on physical and chemical properties of the enzyme. 3. Elucidate a partial amino acid sequence of the protein. This study is relevant to understanding the mechanisms of genetic regulation in higher organisms which are responsible for the complex processes of differentiation and development.